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Int 2Fh / 4B01h         Build Notification Chain                     DOS 5.0+

    Creates a linked list of notification-function handlers for global
    client programs and for client programs running in the current
    session. The task switcher calls this function to determine which
    client programs are to be notified about changes to the session.
    To receive notification, client programs must intercept Interrupt
    2Fh and process Build Notification Chain when they receive the
    function call.

        Note    Use the task switcher API patch

        Entry   AX    = 4B01h
                ES:BX = 0000:0000h
                CX:DX = Address of service-function handler

        Return  ES:BX = Pointer to a SwCallBackInfo structure
                or
                ES:BX = 0000:0000h   No notification

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